This invention is a method for making containers from multilayer billets with minimal scrap loss.
Containers may be formed by various methods. For example, containers may be formed from "billets"--defined as pieces of material cut from sheet, the perimeter of which is cut to match the outer dimensions of the container.
When containers are thermoformed, and punched or cut out of a multilayer sheet of thermoplastic material, a residual "web" of the uncut material typically represents 35% to 50% of the original uncut sheet.
The resulting web is considered "scrap" and is ground down and referred to as "regrind" and sold, usually at only a fraction of the cost of the materials in the scrap. The scrap is virtually useless. Both the barrier material and the glue are several times more expensive than polypropylene. So it is desired to result in a monolayer web of scrap which does not contain expensive materials. Once reground, the scrap itself, even though it contains expensive materials such as barrier materials, is worth less than the original cost of polypropylene. The problem of how to reduce the amount of scrap generated from conventional thermoforming methods is one that has plagued the industry.
Often it is desirable to coextrude several layers of sheet materials together before cutting out billets. This is to produce a multilayer billet which, when formed, results in a multilayer container. For example, the container could contain layers of polypropylene/tie layer/barrier layer or barrier layer adhesive/polypropylene as just one of many combinations. Barrier layers are quite desirable in many containers.
One of the reasons that "wasting" of the web of scrap is so detrimental is that oftentimes the web of scrap contains expensive materials such as barrier layers (EVAL and the like).
In addition, the time involved in grinding the resulting web of scrap is burdensome since it cannot be reused to make billets.
A method of producing multilayer containers which are thermoformed in a conventional manner utilizing conventional roll fed sheet, is desired where there is no resulting web of multilayer scrap. This has been accomplished by applicant in this invention. Here, the resulting web of monolayer scrap may be easily mixed and used again and again.